1. Field of the Invention
The present invention relates to a process for reducing the viscosity of sulphur-containing heavy oil. The process is intended to be practised while the heavy oil is in place in a subterranean reservoir and is undergoing steam injection. The invention involves introducing one or more specific metal ions into the reservoir, to obtain an incremental reduction in the viscosity of the oil, over that obtained from the heating effect of the steam. In a preferred feature, carbon monoxide (CO) is introduced with the metal ions into the hot reservoir to cause a still further reduction in oil viscosity and improvement in recoverability with selected metal ions.
2. Description of the Prior Art
"Heavy oil" is a term commonly applied to describe oils having a specific gravity less than about 20.degree.API. These oils, which include tar sand bitumen, are not readily producible by conventional techniques. Their viscosity is so high that the oils cannot readily be mobilized and driven to a production well by a pressure drive.
To produce such oils, the industry has resorted to first heating them in situ, to reduce their viscosity to a level where the oil is amenable to displacement. A pressure drive is then used to force the oil through the reservoir to the production well.
One such heating procedure involves injecting steam into the formation. The present invention is practised in conjunction with such steam injection.
Much work has heretofore been carried out to identify suitable additives which could be introduced into a heavy oil reservoir with steam, to obtain an incremental reduction in oil viscosity. In general, this work has centered on the use of solvents, surfactants and gases (such as CO and CO.sub.2), as the additives.
We have chosen to try the approach of attempting to chemically induce the fragmentation in situ of the hydrocarbon macromolecules which make up heavy oil. As a first step in this direction, we accepted the widely held view that heavy oil macromolecules are composed of long chains of aliphatic and aromatic groupings linked together, at least in part, by sulphur atoms. We then sought to ascertain an additive which, in an environment such as would exist in a heavy oil reservoir undergoing steam injection, would seem to be capable of selectively cleaving the macromolecules at the sulphur bonds.